The present invention relates to a method for converting line sequential color signal to simultaneous signal, wherein line sequential carrier color signal is frequency-demodulated, followed by amplitude modulation and conversion to simultaneous signal, to obtain standard signal for NTSC system, providing a useful method for reproducing carrier image signal recorded on magnetic recording media.
France and other European countries use a standard television system called SECAM System. In SECAM system, two color difference signals R-Y and B-Y are selected by switching with a line sequentializing switch alternately every one horizontal scanning period (hereinafter called "1H period"), thus formed line sequential color difference signal is frequency-modulated with a color difference subcarrier and overlapped with luminance signal to obtain carrier image signal.
When reproducing the carrier image signal, after demodulation the signal is delayed by a 1H period, the delayed signal and undelayed signal are parallelly taken out to fill the removed portion of the color difference signal every 1H period to obtain two continuous color difference signals R-Y and B-Y. The process of taking out two color difference signals parallelly from line sequential color difference signal is referred to "simultaneization". The major components for the simultaneization are the 1H period delay circuit and the simultaneizing switch which alternately takes out the signal delayed by the 1H period delay circuit and undelayed signal.
Owing to its frequency modulation system, the SECAM system has a wide adaptability, especially when the time axis fluctuates as in the case of magnetic recording and reproduction system. In this case, because the line sequential color difference signal which is sequentialized at the recording side is frequency- modulated, the line sequential carrier color difference signal must be once demodulated then simultaneized, and the simultaneized color difference signal must be balance-modulated again in order to obtain standard signal for NTSC system at the reproducing side.
FIG. 1 is a block diagram showing schematically a device of a conventional technology in which standard simultaneous signal for NTSC system is obtained at the reproducing side of the magnetic recording and reproducing system based on SECAM system. As shown in the figure, a line sequential color difference signal LSS which is demodulated by a frequency demodulator and supplied through an input terminal 1 is applied to a simultaneizing switch 3 directly or through a 1H period delay circuit 2.
The simultaneizing switch 3 contains a switch 4 which outputs a color difference signal R-Y and a switch 5 which outputs another color difference signal B-Y. The switch 4 has contacts 4a and 4b, and the switch 5 has contacts 5a and 5b. The switches are switched at every 1H period by a control pulse Pc which is supplied through an input terminal 6 so that the contacts 4a and 5b, and 4b and 5a respectively, are selected simultaneously. The control pulse Pc can be easily generated by a horizontal drive signal (HD pulse).
Balanced modulators 7 and 8 balance-modulate subcarriers SC1 and SC2 which are different in phase by 90 degrees each other with the two simultaneized color difference signals R-Y and B-Y, respectively. Thus, the color difference signal R-Y balance-modulates the subcarrier SC1 which is produced by advancing 90 degrees the phase of the subcarrier SC2 supplied through an input terminal 9 by a phase shifter 10, and the color difference signal B-Y directly balance-modulates the subcarrier SC2, respectively.
A mixer 11 mixes two simultaneized carrier color difference signals R-Y' and B-Y' obtained in the balanced modulators 7 and 8 and transmits a carrier color difference signal CHROMA which is a color signal component of the NTSC standard signal through an output terminal 12.
13 and 14 indicate clamp circuits provided at the preceding stages of the simultaneizing switch 3, and 15 and 16 indicate clamp circuits provided at the following stages of the simultaneizing switch 3 (preceding stages of the balanced modulators 7 and 8), respectively.
The necessity of providing the clamp circuits 13 and 14 at the preceding stages of the simultaneizing switch 3, in addition to the clamp circuits 15 and 16 provided at the preceding stage of the balanced modulators 7 and 8, is described below.
In the simultaneizing system described above, the balanced modulators 7 and 8 are supplied with a through signal and a signal passed through a 1H period delay circuit 2 alternately every 1H period. However, since the delay circuit 2 substantially damps not only the transmission rate but also the signals, and since offset voltages of the analog switch 3 at contacts a and b are different each other, the balanced modulators are supplied with signals having different DC level at every 1H period. If there are no clamp circuits 13 and 14 at the preceding stages of the simultaneizing switch 3, time constants of the clamp circuits 15 and 16 must be increased sufficiently to clamp the average level of the signals. Therefore, if a change occurs in image pattern of the line sequential color difference signal LSS, its average level will also fluctuate and poor modulation in the balanced modulators 7 and 8 will result in.
In order to eliminate such a problem, the time constants of the clamp circuits 15 and 16 must be increased so that the blanking level of the color difference signal can be clamped and, at the same time, the clamp circuits 13 and 14 must be installed at the preceding stages of the simultaneizing switch 3, as shown in FIG. 1, resulting in complicated circuit configuration.
Another major defect is that the simultaneizing switch 3 must be an analog switch for switching the line sequential color difference signal LSS, and actual circuit is extremely complicated as shown in FIG. 2.